The invention relates to a method for determining material parameters of a doped semiconductor substrate by measuring photoluminscent radiation.
In doped semiconductor substrates, particular in silicon wafers, it is known to obtain conclusions about the material parameters of the semiconductor substrate based on a luminescent radiation generated in said semiconductor structure and its measurement. In particular the measurements of the luminescent radiation are used to gather information about the material quality with regards to the effective life time of the minority charge carriers or the effective duration of diffusion corresponding thereto.
Here, it is known to determine the diffusion length/life time of the semiconductor material from the measurement of the luminescent radiation. Respective methods are described in literature, for example in Würfel, P. et al., “Diffusions Lengths of Silicon Solar Cells from Luminescence Images”, Journal of Applied Physics, 101; 123110, 2007, or in Trupke, T. et al, “Photoluminscence Imaging of Silicon Wafers”, Applied Physics Letters, 89: 044107, 2006.
Furthermore, measuring methods are known in which the impinging of the semiconductor substrate occurs with modulated excitation radiation so that the rate of generation G of charge carrier pairs in the semiconductor substrate, generated by the excitation radiation, is temporarily inhomogeneous and shows at least one maximum. Typically a periodic, sinusoidal modulation is used. Here the modulation occurs such that a quasi static condition is achieved. The respective measuring method with an assessment of the photoluminescent radiation is known as the quasi steady-state photoluminescence life time measurements (QSS-PL) and described for example in T. Trupke and R. A. Bardos, 31st IEEE PVSC, Orlando, 2005.
In the quasi steady-state photoluminescence life time measurement methods the quantitative determination of the excess charge carrier density Δn is essential to determine for example the effective life time τ of the minority charge carriers. For calibration of the quasi steady-state photoluminescence measuring methods it is known to determine respective calibration parameters via the self-consistency method, such as described in T. Trupke, R. A. Bardos, M. D. Abbott, “Self-Consistent Calibration of Photoluminescence and Photoconductance Lifetime Measurements”, Applied Physics Letters, 87: 184102, 2005.
In this self-consistent calibration the knowledge of the concentration of electrically active doping atoms N in the semiconductor substrate (in the following called doping concentration for reasons of simplification) is necessary as well as the time-dependent generation rate G(t) during the impingement of the semiconductor substrate with respectively modulated excitation radiation, here quantitatively. Here, particularly the determination of the doping concentration N with conventional methods is problematic (such as an eddy-based measuring method, cf. Sinton. R. et al, “Quasi Steady-State Photoconductance, a new method for solar cell material and device characterization”, 25th IEEE PVSC, Washington D.C., 1996). Presently the knowledge becomes generally accepted in professional literature that in many novel materials for the solar cell production, such as in multi-crystalline silicon, conventional methods for the determination of the doping concentration N reach their limits, because they require the precise quantitative assumptions regarding the mobility of the charge carriers, which cannot be obtained here (cf. Cuevas, A. “The paradox of compensated silicon”, COMMAD IUMRSICEM08, Sydney, 2008).